This invention relates to a metrological apparatus, and more particularly to apparatus of the type employed for automated optical inspection of manufactured objects, particularly in regard to the determination of precise dimensional measurements of such objects. Even more particularly this invention relates to an improved variable magnification optical system and associated video camera for accurately performing computerized image analysis and dimensional measurements of such objects.
Metrological apparatus of the type described normally includes a support for the object upon which dimensional measurements are to be made, and some means for precisely moving either the object or the associated video camera which is employed for recording or displaying a magnified image of the object that is being inspected. Such apparatus in addition to being able to perform measurements in a horizontal or X-Y plane, also may include autofocus means for determining heights of the object in the Z direction normal to the X-Y plane. In this manner such apparatus is capable of producing a full, three-dimensional inspection of the object.
Heretofore, as described for example in U.S. Pat. No. 5,389,774, it has been customary normally to employ in a video inspection system of the type described a conventional zoom lens mechanism to vary focal length and to maintain focus. Typical such zoom lenses are not telecentric, so that the position of the object within the depth of focus significantly affects apparent size of the object, particularly at lower magnifications and at larger fields.
One object of this invention is to provide for an inspection system of the type described a telecentric zoom lens device that covers a broad range of magnifications, has a large collimated space for insertion of an illumination means, has large working distance, and allows for use with interchangeable infinity corrected objectives.
Another object of this invention is to provide a telecentric zoom lens device of the type described having an F-number or aperture which is intentionally nearly constant at the image, thereby presenting uniform intensity and resolution throughout the magnification range.
Other objects of the invention will be apparent hereinafter from the specification and recital of the appended claims, particularly when read in the light of the accompanying drawings.